The rapid development of optical fibers, which permit transmission over long distances and at high bandwidths, has revolutionized the telecommunications industry and has played a major role in the advent of the information age. However, there are limitations to the application of optical fibers. Because laying optical fibers in the field can require a large initial investment of time and material, it is not cost effective to extend the reach of optical fibers to sparsely populated areas, such as rural regions or other remote, hard-to-reach areas. Moreover, in many scenarios in which a business may want to establish point-to-point links among multiple locations, it may not be economically feasible to lay new fibers.
On the other hand, wireless radio communication devices and systems provide high-speed data transmission over an air interface, making it an attractive technology for providing network connections to areas that are not yet reached by fibers or cables. Wireless communications are rapidly carried through the air and space by electromagnetic signals, generally from one antenna to another antenna. However, currently available wireless technologies for long-range, point-to-point (or point-to-multipoint) connections of electromagnetic signals encounter many problems, such as limited range and poor signal quality.
An antenna is responsible for transmitting or receiving signals that carry information, specifically electromagnetic signals such as microwave, radio or satellite signals, across air and space from one place to another place. An antenna is generally used with other components as part of an antenna system to accomplish its tasks. An antenna functions by changing the form of the signals, making them accessible for human use. Electromagnetic signals in the form of electromagnetic waves are transmitted (delivered or sent) from one antenna and are received (picked up) by another antenna. Electromagnetic waves are complex and have both an electric component and a magnetic component. One antenna transmits signals by converting an electrical current into electromagnetic waves (such as radio waves) that proceed out from the antenna into air and space. Some of the electromagnetic waves (such as the radio waves) are received by another antenna which converts them back into an electrical current. There are many types of electromagnetic waves, and a particular antenna system is designed to work with a particular type of waves. Radio frequency (RF) and microwave antennas represent a class of electronic antennas designed to operate on wireless electromagnetic signals in particular ranges, the megahertz to gigahertz frequency ranges. Conventionally these frequency ranges are used by most broadcast radio, television, and other wireless communication (cell phones, Wi-Fi, etc.) systems with higher frequencies often employing specialized antennas, called parabolic antennas. (Although certain wavelengths of electromagnetic radiation are referred to as “radio waves” they carry, in addition to signals for AM or FM radio, signals for cell phones, televisions, etc.). The suitability of a particular antenna system for a given purpose is determined by the antenna's frequency, gain, and beam width. In some cases, an antenna may transmit and/or receive signals such as microwave, radio or satellite signals from a second antenna. Although any given antenna is generally capable of both delivering and receiving a particular type of electromagnetic signals, in some cases, an antenna system may be configured so that an antenna is only responsible for delivering or receiving electromagnetic signals, but does not do both.
An antenna system may use a reflector to direct electromagnetic radiation from the air or space to an antenna. One familiar type of reflector is a parabolic reflector. A parabolic antenna is an antenna that uses a parabolic reflector which is a curved surface with the cross-sectional shape of a parabola, to direct electromagnetic signals (e.g., radio waves) in a particular direction so they are better able to be picked up by the antenna. A parabola is a symmetric curve and a parabolic reflector is a surface that describes a curve throughout a 360° rotation, a shape referred to as paraboloid. Conventionally, a parabolic antenna has a portion shaped like a dish and so is often referred to as a “dish antenna” or simply “a dish”. A parabolic reflector is very effective at directing waves into a narrow beam. In particular, and as indicated above, a parabolic reflector is very effective at reflecting waves into collimated plane wave beam along the axis of the reflector. Parabolic antennas systems are generally used for long distance communication between buildings or over large geographic areas.
Parabolic antennas provide for high directivity of the radio signal because they have very high gain in a single direction. In other words, the signal can be sent in a desired direction, such as radiating outwards towards other antennas rather than being sent upward into space where there are no antennas. Beam width is a measurement of the area over which the antenna receives signal and is important in determining how well an antenna functions. To achieve narrow beam-widths, a parabolic reflector must typically be much larger than the wavelength of the radio waves used, so parabolic antennas are typically used in the high frequency part of the radio spectrum, at ultra-high (UHF) and super high (SHF; e.g., microwave) frequencies, where the wavelengths are small enough to allow for manageable antenna sizes. Parabolic antennas may be used in point-to-point communications, such as microwave relay links, WAN/LAN links and spacecraft communication antennas.
The operating principle of a parabolic antenna is that a point source of radio waves at the focal point in front of a parabolic reflector of conductive material will be reflected into a collimated plane wave beam along the axis of the reflector. Conversely, an incoming plane wave parallel to the axis will be focused to a point at the focal point.
Conventional radio devices, including radio devices having parabolic reflectors, suffer from a variety of limitations and problems. For example although a wireless signal of interest has to be received by an antenna to be useful, an antenna does not just receive a specific signal of interest, but it receives any signal that comes its way (provided that the signal meets certain criteria regarding wavelength, etc.). Other difficulties and limitations include aligning with an appropriate receiver, monitoring and switching between transmitting and receiving functions, avoiding interference (including reflections and spillover from adjacent radios/antennas), loss of signal, mechanical damage, expense, and complying with regulatory requirements without negatively impacting function. Described herein are devices, methods and systems that may improve wireless communication devices and address issues such as those identified above. In particular, described herein are apparatuses that may provide isolation of an emitted beam by selectively attenuating portion of the emitted signal.